The number of antigen sites on red cells varies. Lower number of antigen sites lead to weak or missing reactions with anti-A and anti-B. Subgroups of A are more common than subgroups of B.

The two most common subgroups of A are A1 and A2. Both have strong reactions with anti-A reagent. Anti-A1 lectin is used to identify A1 and A2 cells. Anti-A1 lectin (prepared from Dolichos bifloris seeds) reacts with A1 but not A2 red cells. While anti-A1 may be found as an alloantibody in the serum of group A2 individuals, it is more commonly found in group A2B individuals.

Mixed-field agglutination is a characteristic of A3 red cells with anti-A. A mixed-field reaction is defined as the appearance of strongly agglutinated cell clumps admixed with unagglutinated cells. A microscopic representation of mixed-field agglutination is shown on the right.

Generally, A subgroups do not pose a problem on the blood bank bench unless the patient demonstrates naturally occurring anti-A1 or has been stimulated (by transfusion or pregnancy) to produce anti-A1. However, Anti-A1 is considered clinically insignificant unless there is reactivity at 37° C. Patients demonstrating this reactivity pattern will have ABO testing that resembles the following:

Anti-A

Anti-B

A1 cells

B cells

Antibody Screen

Patient Results

3+

0

1+

4+

Negative

In this case, we have a discrepancy between the forward and reverse types - the forward type potentially having an A subgroup while the reverse type demonstrates an antibody to A1 reagent red blood cells.